Confused and frustrated! Am I doing something wrong or trying to do something that’s not possible?

I’ve had Trainerroad for over a year now and really enjoyed it. I’ve just changed turbo to a Tacx Bushido. I also have a Stages power meter.

The Bushido works fine in Erg mode on its own in Trainerroad, but I’d like to control it with the Stages so I’m using the same power readings when riding inside and outside. I think this should be possible but cannot get it to work and I don’t know if the problem caused the Bushido, Trainerroad or my incorrect settings and hope someone can point me in the right direction.

If I start a session in Erg mode controlled by the Stages I get a couple of minutes in and the turbo slams on the brake and sets the resistance so high it is almost impossible to ride. To overcome this, I’ve tried doing the warmup in resistance or standard (when I tried last night standard mode wasn't available?) mode and then switching to Erg at the start of the first interval. Rather than tracking the power as per the target the resistance then sets to a level 40 to 50 watts higher than required for the first minute or so of the interval before dropping.

I’ve currently got the Trainerroad set to use power meter to control electronic trainer and tried different smoothing time. Is there something else I need to set on Trainerroad.